1) Field of the Invention
The invention relates to a rotor blade for a fan. Such fans are used for cooling and have a typical diameter which could amount up to 10 meters. Such fans comprise a hub on to which several rotor blades are mounted. Concentrically to this hub a housing is arranged, to direct the airflow which is generated by the rotation of the rotor blades.
2) Description of the Prior Art
The aerodynamic pressure generated locally by a rotor blade is proportional to the lift coefficient and to the square of the airspeed. At the tip, i.e. the free-end of the rotor blade, the aerodynamic pressure could be as high as 1500-2000 Pa.
The clearance between the tip and the housing is kept small in order to avoid xe2x80x9cleakagexe2x80x9d of the fan. As this tip clearance is small, the aerodynamic pressure generated at the tip of the blades is reacted onto the housing on a certain defined area. This area is proportional to the cordlength of the rotor blade and to the thickness at the tip of the blade. As the rotor blade rotates, and the housing is stationary, a high amplitude pulsating force is exerted onto the housing. This can result in fatigue cracks and/or failure of the housing, premature wear of the fan drive system and additional noise.
According to the prior art the disadvantages are at least partly eliminated by providing a stiff housing and/or support structure, or by increasing the number of fan blades. Both solutions result in an increase of the costs of a cooling unit.
It is therefore an object to provide a low cost solution relative to the solutions of the prior art.
This object is achieved by a rotor blade according to the invention, which blade comprises a first blade zone having an airfoil cross-section and a second blade zone having a cross-section of reduced thickness, wherein the average thickness of the second blade zone is less than 50% of the average thickness of the first blade zone.
By reducing the average thickness of the second blade zone, the area is reduced onto which the aerodynamic pressure is reacted. Therefore the pulse force is reduced which results in improvement of the fatigue life of the housing, reduction of the vibrations, reduction of the wear of the fan drive system and provides a solution at reduced costs.
In an embodiment of the rotor blade according to the invention, the length of the second blade zone is less than 15% of the length of the first blade zone.
Preferably, the length of the second blade zone is in the range of 0,05-0,6 of the average chord length of the first blade zone. Tests have revealed that these features provide for the desired reduction in the force exerted onto the housing, while keeping a sufficient performance.
In another preferred embodiment of the rotor blade according to the invention, the average thickness of the second blade zone is in the range of 0,02-0,2 of the average thickness of the first blade zone.
The thickness of a rotor blade is defined as the maximum thickness of a transversal cross-section of the rotor blade.
In again another preferred embodiment of the invention the mean line of the cross section of the second zone is substantially parallel to mean line of the cross section of the first zone. This provides for a similar air flow over the full length of the rotor blade.
In again another embodiment of the rotor blade according to the invention the upper surface of the second blade zone is substantially in line with the upper surface of the first blade.